In recent years, much research in the field of implantable biomedical devices is being carried out for wide range of applications such as neural recording, cochlear implant, retinal prosthesis, and etc.
One of the issues in the development of implantable biomedical devices is the consistent provision of a stable, reliable power supply. The usage of batteries is avoided since it requires periodic replacement due to its limited lifetime and can cause much discomfort and health risk to the patient with the implant device. Therefore, wireless power transfer to implantable biomedical devices are used as it obviates the need for implanted batteries or pierced wirings and can provide a safer and more robust implementation.
FIG. 23 shows a conventional wireless power transfer system 2300 for biomedical implants. The conventional wireless powering link 2300 has an inductive coupling 2302, a rectifier 2304 and a regulator 2306. Inductive coupling is generally used for transcutaneous power transfer from the outside body into the implanted device wirelessly. The inductive coupling 2302 includes a primary coil 2308 of an external transmitting device 2312 on an external side 2310 of the conventional wireless power transfer system 2300 and a secondary coil 2314 of an implant device 2318 on an implanted side 2316 of the conventional wireless power transfer system 2300. The primary coil 2308 and the secondary coil 2314 are coupled at a distance with living tissue in between.
It is desirable to maximize the efficiency of the wireless power transfer system 2300 so that less transmitting power may be required and/or a longer distance between the external transmitting device 2312 and the implant device 2318 can be facilitated while using the same transmitting power from the external device 2312. An operating frequency for power transfer is preferable to have a relatively high value, considering the size of the secondary coil inductor 2314 in the implant device 2318. However, the operating frequency cannot be too high due to the increased tissue absorption. High efficiency is also desirable for living tissue safety by reducing the RF exposure to avoid cellular damage. However, the overall efficiency of the conventional wireless power transfer system 2300 is generally far less than 10%. The inductive coupling 2302 generally has quite a low efficiency on account of unfavorable coupling conditions such as size constraint, power requirement and biocompatibility.
The rectifier 2304 of the conventional wireless power transfer system 2300 is utilized to convert a transmitted AC signal to an unregulated DC signal. The unregulated DC signal can be applied to the regulator 2306 to obtain a stable DC voltage which is supplied to the building blocks in the implant device 2318.
Different types of rectifiers can be used for the rectifier 2304. CMOS rectifiers have the advantage of its low-cost process and its compatibility with other building blocks. However, CMOS rectifiers may have a problem of efficiency degradation due to the forward voltage drop and reverse-leakage current. In comparison with CMOS rectifiers, the Schottky diode rectifier may require additional process steps that lead to an increase in the implementation cost.
Moreover, the efficiency problem of conventional wireless powering links becomes more significant considering large power dynamic range. The required regulated DC output power is application-specific, which can vary from microwatt to watt level in biomedical applications. In high power case, big transistors required in the rectifier 2304 to reduce its forward voltage drop may impose the heavy loading effect on the conventional efficiency-boosting structure of the inductive coupling 2302, which adds a capacitor to form the high-Q resonance. In low power case, the reverse leakage current of the rectifier 2304 may degrades its efficiency, and the regulator 2306 may require more power consumption or off-chip capacitors to keep its stability, which is not desirable for size and cost considerations.